The present invention relates to an improved process for controlling the pressure in a processing tank or vessel used for heating and cooling deformable sealed packages having volatile contents. The invention relates more particularly to complete sterilization processes, such as retorting and autoclaving when sterilizing food, medical products or industrial products, or partial sterilization processes such as pasteurization of food or other products.
Retort sterilization of food has been an important method of food preservation for many decades. The well known process of canning is the retort sterilization of cans and jars, which are substantially non-deformable under typical retort conditions. The equipment and technology for canning is well known.
In the mid-1960's, the retort punch was commercialized in Japan for such food products as curry and stew. These pouches were essentially heavy-walled sealed plastic pouches. The retort pouch was not successful with the consuming public of the United States at that time. However, the military has used retort pouches for "MRE" or "meals ready to-eat" since the early 1970's.
The retort sterilization of food in pouches presents a significant problem for the packaging industry; namely, the pouches are prone to damage during the sterilization process. This problem is partially overcome by making the pouches stronger and more durable. Thus, one approach to retort sterilization of a pouch is to make the pouch stronger and more like a can or a jar. Of course, doing so comes at the expense of greater package cost and weight.
In the 1980's, American food companies began to investigate the use of "retortable" plastic trays, bowls, cups and other formed containers with peelable lids for shelf-stable foods. The use of plastic and other deformable food containers arose in wake of the advent of microwave ovens. Food contained in plastic retort containers can be taken off the shelf and heated in the microwave oven without the need for transferring the contents into a separate bowl or dish for heating. The popularity of these retortable plastic food containers is expected to continue its growth. However, maintaining the shape and integrity of the container and lid is essential to providing a product that is acceptable to consumers.
The problem with plastic retortable food containers is that they usually contain a certain volume of air or gas in the area above the contents, commonly referred to as headspace. When a retortable container containing food and air is heated, the air and the water vapor pressures inside the package increase. If these pressure changes are not offset properly by keeping the processing tank or vessel pressure similar to the pressure within the package, the package expands and may explode upon retorting.
Conversely, when an air-containing heated retortable container is cooled (whether after a heating step or independently), the air pressure and water vapor pressure decrease. If the pressure inside the processing tank is not reduced accordingly, the retortable container will contract and may collapse.
Thus, the retort sterilization of food in a deformable plastic container while maintaining the shape and integrity of the plastic container is a difficult task which requires precise pressure and temperature control inside the processing tank or retort chamber.
Similar problems are faced during the autoclave sterilization of medical products in deformable containers. For example, common medical applications include the sterilization of intravenous solutions, water, formulas, drugs, and liquids to be injected via syringe or other means.
Comparable problems are encountered in partial sterilization processes, and heating processes generally. The most common partial sterilization application is pasteurization. It is therefore to be understood that when the term "retort" is used, any one of the terms "pasteurize", "autoclave", or "sterilize" may be readily interchanged.
A retort or autoclave sterilization process comprises three phases: the "come-up" or heating phase, the processing (or "sterilization") phase, and the "come-down" or cooling phase.
Proper pressure control is critical to maintaining the shape and integrity of deformable packages containing headspace, especially during the "come-up" and "come-down" phases. For the purposes of the discussion below, the "gas" which fills the headspace may be nitrogen, carbon dioxide, and/or air. "Volatile material" is the water or other liquid components of the package contents which are volatile at the processing temperatures. (For most foods, the volatile material is water; however, for others it may be a mixture of components of varying volatility). For packages containing an industrial or medical devices, it may be alcohol or some other volatile material, preservative, or coating material.
The "come-up" phase is the time period in which the heating medium of the processing tank is being heated to the prescribed pasteurization or sterilization temperature. During this time, the contents of the package are also being heated and the air and water vapor pressures inside the package are increasing. The pressure inside the processing tank must be increased or the package will expand and may explode. But if the pressure in the tank is increased too much or too quickly, the package will contract and may collapse.
The processing phase is the time period in which the heating medium of the processing tank is maintained at the prescribed temperature so that the contents of the package can be sterilized. In the food industry, the degree of sterilization is described in terms of F.sub.o, which represents the accumulative time in minutes that the product was exposed to thermal processing equivalent to 250.degree. F. The F.sub.o is typically calculated at the lowest product temperature during thermal processing.
Most food retort processes use an F.sub.o significantly greater than the theoretically required value in order to assure safety. However, F.sub.o is only a theoretical standard for the sterilization of a product, and it is important to ascertain the actual level of sterilization achieved for each product using a decay test, a microbe test, or the like. In other words, the sterilization of each product should be experimentally verified before mass production of that product.
During the processing phase, the internal package or container pressure continues to increase for several minutes as the temperature of all the air and other contents of the package reach thermal equilibrium. The pressure inside the processing tank must be adjusted accordingly.
The "come-down" phase is the time period in which the heat transfer medium (usually water) and therefore the product is cooled. As the heat transfer medium is cooled, the package is cooled and the contents of the package are cooled. Consequently, the air and water vapor pressures inside the package decrease during this "come-down" phase. The pressure inside the processing tank must also be reduced or the package will contract and may collapse. However, if the tank pressure is decreased too much or too quickly, the package will expand and may explode.
Several schemes have been established to avoid the consequences of too little or too much pressure in the processing tank. Those schemes include vacuum sealing to reduce the internal air pressure; eliminating the head space in the package to an absolute minimum so as to prevent expansion of air during retorting; filling the package with heated food or contents to reduce internal pressure in the package during the processing phase; using very heavy-gauge trays and lids to withstand the pressure differential; using ultra-strong peelable seals to withstand the internal pressure in the trays; and, using lower retort temperatures and longer retort times. All of the above methods have shortcomings for the retort sterilization of a lightweight, deformable tray with a peelable lid.
Other schemes attempt to provide control over the retort vessel pressure in a manner intended to minimize the opportunity for package deformation or failure. One such scheme is the so-called "dummy method." In this scheme, a small retort chamber with a window is kept at the same temperature and pressure as the processing tank. The pressure in the processing tank is controlled on the basis of the visually observed state of deformation of the model package kept in the small chamber. It has been found, however, that a high level of skill is required for visually discerning the state of deformation of the model package in the small chamber. Furthermore, the additional small chamber requires modification of existing equipment, and a model package must be used during each sterilization cycle.
Yet another scheme is taught by Sugisawa et al., U.S. Pat. No. 4,874,580, incorporated herein by reference. Sugisawa teaches that a simple delay in the increase of pressure in the processing tank during the "come-up" phase will prevent the package from collapsing. Also, it teaches that another delay in the lowering of pressure in the processing tank during the "come-down" phase will prevent the package from exploding.
More specifically, Sugisawa teaches a two-step method. First, the pressure change pattern of the air-containing retortable package is determined by installing a thermocouple or other temperature probe inside the package. The internal package pressure is calculated according to the following equations: EQU P.sub.c P.sub.a +P.sub.w ( 1)
where P.sub.c is the internal pressure of the package; P.sub.a is the partial pressure of air inside the package; and P.sub.w is the partial saturated water vapor pressure inside the package. The partial pressure of air, P.sub.a, may be calculated at any air temperature using the ideal gas law as shown in Equation No. 2: EQU PV/T=Constant (2)
where P is the absolute pressure of air or gas; V is the volume of air or gas; and T is the absolute temperature of the air or gas. Because the term PV/T of Equation No. 2 is constant, for the contents of any sealed container having initial conditions P.sub.1, V.sub.1, and T.sub.1 and modified conditions P.sub.2, V.sub.2, and T.sub.2, EQU P.sub.1 V.sub.1 /T.sub.1 =P.sub.2 V.sub.2 /T.sub.2 ( 3) EQU and, EQU P.sub.2 =P.sub.1 V.sub.1 T.sub.2 /V.sub.2 T.sub.1 ( 4)
The conditions of the package at the time it was sealed are known, and they are denoted with the subscript "p" (i.e., P.sub.p, T.sub.p, P.sub.wp etc.). Furthermore, the volume of headspace does not vary appreciably if the tank pressure and the internal container pressure are balanced; therefore the volume terms cancel. Using P.sub.a and T.sub.a as the variable pressure and temperature terms for the headspace, the gas pressure of a sealed package is expressed as: EQU P.sub.a =P.sub.p T.sub.a /T.sub.p ( 5)
Substituting the terms of Equation No. 5 into Equation No. 1, EQU P.sub.c =P.sub.p T.sub.a /T.sub.p +P.sub.w ( 6)
Correcting for the partial pressure contributed by any water vapor present at the time the package was sealed, Equation No. 6 becomes EQU P.sub.c =(P.sub.p -P.sub.wp)T.sub.a /T.sub.p +P.sub.w ( 7)
Thus, in accordance with the teachings of Sugisawa, Equation No. 7 represents the current method used to estimate the internal package pressure, P.sub.c, of a sealed package during a retort or autoclaving process. However, as described below, Equation No. 7 provides only an approximation of the actual internal pressure during the retort cycle.
One general object of the invention is to provide a process for sterilizing or pasteurizing the contents of a sealed deformable package with a peelable lid, or of a peel-open pouch containing headspace, while maintaining the shape and integrity of the deformable package or pouch.
Another object of the invention is to provide an improved method for controlling the pressure inside a processing tank so that the contents inside a sealed deformable package can be sterilized or pasteurized effectively without adversely affecting the shape and integrity of the sealed deformable package and the peelable lid.
Another object of the invention is to provide an improved method for accurately calculating the partial pressure of water or volatile material vapor inside a sealed deformable package during the "come-up", processing, and "come-down" phases of a retort process.
Another object of the invention is to provide an improved method for autoclaving medical devices or industrial objects in deformable packages with peelable lids, or in peel-open pouches, while maintaining the shape and integrity of the package and lid.
Another object of the invention is to provide an improved method of pasteurizing food in sealed deformable packages with peelable lids while maintaining the shape and integrity of the package and lid.
Yet another object of the invention is to provide an improved method of autoclaving, retorting or sterilizing the contents of a sealed deformable package where a volatile material other than water is present.
Further and additional objects will appear from the description, accompanying drawings, and appended claims.